Electric duct heaters are well known for use in heating air which can be supplied to rooms or spaces through HVAC ducts. The design of these duct heaters is well known and they have been built for many years. They are also regulated for safety performance through UL Standard UL1996. This UL1996 standard is a very stringent standard for practices and evaluation and relates to the placement of duct heaters at a particular spacing from equipment, structure or the like that is part of or related to the duct containing the electric heater.
Another standard for electric heater applications is UL 1995. This standard controls the electric heater design when it is incorporated into a piece of equipment or other structure that would be near the heater. An example of such a heater would be the heater found in conventional ducted packaged terminal air conditioner (PTAC) units or typical residential HVAC systems. These types of heaters do not necessarily involve the placement of the heater in a duct so that the standards controlling the design for these types of equipment heaters are different than electric duct heaters.
What this means is that not all duct heaters have application as equipment heaters and not all equipment heaters have application for duct heating.
Most present day duct heaters are built with safety limit controls. FIG. 1a shows an example of a typical construction for a duct heater 100 and the limit controls that the heater employs and that uses a one phase layout and one heater coil 102. The limit controls are devices that when heated to a certain point will turn off power to the heater to prevent over temperature or safety problems.
An auto limit 101 will open to turn power off at specified temperatures and automatically close when cooled to some lower temperature, allowing the heater to restart if needed. A backup limit 103 can typically be a one-shot device, meaning this device opens at a specified temperature and does not close again, forcing a technician to replace the device to repair the heater. Also, the backup limit 103 can be a manually-resettable device that opens and can be manually reset by pushing a reset button (typically located on the device) to re-establish power and return the heater to operational status, as needed.
Still referring to FIG. 1a, it should be noticed that the backup limit 103 is typically located toward the top 105 of the heater. The reason for this is that since heat rises, this should be the hottest point in connection with this heating unit if air supply is limited or no airflow exists.
The backup limit 103 is typically set at a higher temperature than the auto limit 101 for cut-oft the idea being the auto limit 101 can open and close at a lower temperature range to keep air supply temperatures lower (if needed), and the higher temperature target of the backup limit 103 would function if the temperature continued to rise for some reason, or if the auto limit 101 fails to function properly, etc.
As shown in FIG. 1b, because of the arrangement of the back up limit 103 and auto limit 101 in the prior art heater 100, flipping the duct heater to accommodate a different installation orientation results in the back up limit 103 being on the bottom 107 of the heater 100, which would be normally cooler than the top 105 of the heater. Thus, this type of heater cannot be used in different orientations and still maintain a safe operation of the heater, particularly with respect to the backup limit functionality.
In order to maintain the functionality of the duct heater for different installations, differently designed duct heaters must be used. This is shown in FIG. 2. As can be seen in FIG. 2, the heater assembly 200 considered as a “left hand” heater is built with the box overhang 201 to the left of the heater 203 (when viewed from the front of the box).
FIG. 2 also shows a heater assembly 205 considered as a “right hand” heater with an overhang 206, which is designed to be inserted into the duct 207 on the side opposite that of the “left hand” heater assembly 200. The backup limit location for heater assembly 200 is at 209 and the backup limit location for the heater assembly 205 is at 211. In order for the backup limit to be located in its desired location, i.e., near the top location, as in the case of the “left hand” heater assembly 200, an entirely different heater must be built if the heater is to be mounted on the other side of the duct, thus the “right hand” design. This is consistent with the state of the prior art, wherein most duct heaters are designated and/or requested as “right” or “left” handed. They are not symmetrical and therefore not flippable in a given installation site.
This also becomes a problem if the installer does not realize the direction of air flow in the duct. Specifying a “left hand” duct heater may not work when the installer realizes that the air is flowing in the opposite direction to his/her understanding and this is affecting duct mounting location, access, or special limitations.
Thus, a need has developed to provide improved heater designs that lead away from the requirement of different heaters for different types of installations or installation characteristics. The present invention is directed to solving this problem.
Another problem with duct heaters is that they are not readily adapted or co-adapted for use for applications that would be required to meet UL 1995 standards and without additional testing. The invention is also directed to this problem and produces a heater design that can be used to meet either UL 1996 or 1995 standards.